1. Field Of The Invention
The present invention relates to piezoelectric ultrasonic transducers, and in particular to such a transducer having broadband characteristics for transmission and/or reception of ultrasonic energy in the medium of air.
2. Description Of Prior Art
The use of piezoelectric transducers as ultrasonic transmission transducers and/or reception transducers in the medium of air is known in the art. Significant problems are associated with ultrasonic energy propagation in air because all materials useful for electro-mechanical energy conversion or mechanical-electrical energy conversion such as, for example, piezoceramic material, quartz, and the like have significantly different intrinsic acoustic impedances in comparison with air and thus the acoustic matching of that material to air is exceptionally poor. Attempts in the prior art to improve such matching have essentially followed one of two paths. Transducers having a particularly high mechanical Q are known which generate very large oscillation amplitudes, and are thus capable of transmitting sufficient energy into the medium of air. The other approach has been that of the so-called film transducer, for example, a capacitor microphone. A good matching of the intrinsic acoustic impedance of the transducer material to that of air is achieved because of the low mass of such film membranes, however, such transducers are extremely susceptible to mechanical damage so that the feasibility of the use of such transducers in an industrial environment is at best limited.
A further approach known in the art is that of utilizing a number of piezoceramic lamellae spaced from each other at distances which are considerably larger than the lamellae thickness, with the lamella being acoustically coupled at one end to a plate which serves as a radiating surface for ultrasonic energy, or as a reception plate for receiving ultrasonic energy. Such a structure is described in German OS No. 28 42 086. With suitable energization of the piezoceramic lamellae the plate of this transducer can be placed in in-phase oscillatory motion, so called piston stroke motion. The electrodes of the lamellae may be connected in parallel or in series as needed.
Whereas the transducer disclosed in OS No. 28 42 086 makes use of lamellae disposed substantially far apart with respect to the thicknesses of the individual lamellae, another approach is to dispose a plurality of individual transducer elements as closely together as possible, so as to mechanically acoustically couple the transducer elements. An example of this type of transducer arrangement is a crystal microphone described in Radio Mentor, Vol. 5 (1950) pages 236-238 or as described in German OS No. 30 40 563. The transducer lamellae are disposed as closely as possible side-by-side or above each other. Such transducer arrangements, particularly the crystal microphone described in Radio Mentor, result in a packet of crystal lamellae with a high cross-coupling, which has a negative affect on the desired piezoelectric efficiency. Moreover, because of the small spacing between the lamellae, only a small volume is available for an electrode or other means for energizing the lamellae, thereby limiting the power output of such devices.